1. Field of the Invention
The present invention is directed to coating materials (pastes) containing PUR dispersions and solvents and to their use for the preparation of water vapor permeable ("microporous") PUR coatings, preferably on textile or leather substrates by the direct or transfer process using the principle of coagulation by evaporation.
2. Description of the Prior Art
Coatings, in particular textile or leather coatings, which have a high permeability for water vapor and are also highly waterproof are of increasing interest, in particular in the clothing and shoe industries.
The advantages of these coatings is that clothing produced therefrom provides optimum protection from water and, in addition, allows body moisture to escape in the form of water vapor. These properties are of great importance not only in terms of health and hygiene but also with respect to wearing comfort.
An important process for preparing microporous, water vapor permeable clothing is the bath coagulation process, which is described, for example, in DE-AS (German Published Specification) 1,270,276 and DE-AS (German Published Specification) 1,769,277. In this process, a substrate is coated with a polyurethane or polyurethane urea dissolved in an organic solvent and the coated product is introduced into a bath containing a non-solvent (for example water) which is miscible with the solvent. The coagulation of the polyurethane takes place by extraction of the solvent by the non-solvent.
The disadvantages of this process are that very large amounts of non-solvent are required for the necessary complete removal of the solvent and that the process is lengthy. Moreover, special, relatively expensive apparatus is necessary for carrying out this process and also for working up the resulting solvent/non-solvent mixtures.
There has not, therefore, been a lack of attempts to prepare water vapor permeable coatings on conventional textile coating machines. The majority of these attempts use the method of so-called coagulation by evaporation. In principle this process is carried out by dissolving the polymer in a volatile solvent, then adding a specific amount of a less volatile non-solvent and coating a substrate with the solution, dispersion or suspension thus formed. The coating is dried by gentle heating, during which the volatile solvent preferably evaporates first. The result is that the polymer coagulates and, after a final drying process, has a microporous structure.
This procedure has been described, for example, in German Patent Specification 1,694,059; the polyurethanes are dissolved in volatile solvents such as tetrahydrofuran or methyl ethyl ketone and then mixed with organic non-solvents having a higher evaporation number, for example petroleum spirits (80.degree.-110.degree. C.).
A similar procedure has been described in Swiss Patent Specification 481,971 which also discloses water as a non-solvent which may be added to the polymer solutions. A further example of this process is the process according to German Patent Specification 2,004,276. In this process, the hydrophilic polyurethanes are based on aromatic diisocyanates and also contain certain percentages of polyoxyethylene compounds as structural components. Solutions of these polyurethanes in certain organic solvents such as methyl ethyl ketone are mixed with water as the non-solvent and applied to a substrate. The coating is then coagulated by selective evaporation and dried.
However, the processes mentioned which operate by the principle of evaporation coagulation still have shortcomings. A substantial disadvantage is that the selective evaporation of the more volatile solvent requires accurate temperature control. The handling of the respective products in the coating machines is therefore complicated and only possible at a relatively low operating speed. A more serious disadvantage, especially with the process disclosed in German Patent Specification 2,004,276, is based on the difficulties processing the polyurethane solutions or suspensions. Although the solids content is low, these products are highly viscous, even before the addition of water which takes place only at the time of application. The products are described as "slurry-like suspensions," and have a high tendency to dry prematurely with the formation of gel particles and specks. They are therefore difficult to handle by the coating machine. In addition, the coatings according to this patent specification have the disadvantage of insufficient light fastness.
It is further known from DE-OS (German Published Specification) 3,610,200 that aqueous dispersions of PUR plastics which carry carboxyl and/or sulphonate groups together with non-polyurethane resins containing pendant and/or terminal carboxyl groups as thickeners (which require ammonia to display their thickening action) and self-crosslinking polysiloxanes and water-immiscible solvents having boiling points of less than 170.degree. C. provide stable pastes, which can be processed on conventional coating machines to give microporous coatings.